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Abstract

This protocol describes the production of GST-Cbx7 fusion proteins from E. coli, originally developed in the recent publication (Zhen et al., 2016). The pGEX-6P-1-GST plasmids encoding the Cbx7 variants were transformed into BL21 competent cells. The fusion protein production was induced by isopropyl-beta-D-thiogalactopyranoside and they were purified by Glutathione Sepharose 4B. This protocol can be adapted for the purification of other proteins.

Polycomb group (PcG) proteins regulate gene expression by modulating higher order chromatin structures (Kerppola, 2009; Simon and Kingston, 2013). PcG proteins are generally found in two major complexes, Polycomb repressive complex (PRC) 1 and 2 (Kerppola, 2009; Simon and Kingston, 2013). PRC2 is a methyltransferase that catalyzes di- and tri-methylation of lysine 27 on histone H3 (H3K27me2/3) (Cao et al., 2002); PRC1 is an ubiquitin ligase that monoubiquitylates histone H2A on lysine 119 (H2AK119Ub) (Wang et al., 2004). Mammalian PRC1 complexes are further divided to canonical and variant PRC1 (Gao et al., 2012, Tavares et al., 2012). Canonical PRC1 is composed of one of each Ring1 (Ring1A/Ring1B), Pcgf (Mel18/Bmi1), Phc (Phc1/2/3), and Cbx (Cbx2/4/6/7/8) proteins. The Cbx family proteins have a conserved chromodomain (CD) that recognizes H3K27me3, suggesting molecular links between the recruitment of canonical PRC1 and H3K27me3 (Blackledge et al., 2015). Recently, we have interrogated the molecular mechanisms underlying the binding of Cbx7-PRC1 to chromatin by live-cell single-molecule imaging (Zhen et al., 2016). We showed that the CD and AT-hook-like (ATL) motif of Cbx7 constitute a functional DNA-binding unit by electrophoretic mobility shift assay (Zhen et al., 2016). Here, detailed conditions are presented which allow the production of GST-Cbx7 fusion proteins from E. coli. With modifications, this protocol may be used for the purification of other proteins. The purification of fusion proteins by GST fusion system has been widely applied in various biochemical and structural studies (Harper and Speicher, 2011).

Note: The experimental procedure was revised from the published protocol (Harper and Speicher, 2011).

Expression of GST fusion protein

Transform competent BL21 cells with pGEX-6P-1-GST plasmids that encode the Cbx7 variants by incubating plasmid with cells on ice for 10 min, then heat shock at 42 °C for 45 sec. The mixture was put on ice for 2 min, LB medium was added, and the mixture was incubated for 1 h at 37 °C while shaking at 250-300 rpm. After that, the cells were spread onto agar plate containing ampicillin.

Incubate the culture at 37 °C while shaking at 250-300 rpm until the OD600 is 0.5-0.7 (Note 1).

Induce the protein expression by adding IPTG to a final concentration of 1.0 mM (stock concentration: 100 mM, the powder was dissolved in MilliQ water and the solution was filtered before used) and incubating at 37 °C while shaking at 250-300 rpm for 5 h.

Harvest cells by centrifugation at 4,000 x g for 20 min at 4 °C.

Carefully decant the supernatant, leaving about 15-50 ml in the centrifuge bottle.

Resuspend the cells and transfer to a 50 ml conical tube and centrifuge at 4,000 x g for 20 min at 4 °C.

Decant the supernatant (Note 2).

Sonication (Note 3)

Resuspend the cell pellet in 25 ml of lysis buffer.

Lyse cells by sonication at 4 °C using the standard probe with the following settings:

15-sec on

45-sec off

45% input (45% amplitude)

6 min: total time on

Purification

To the mixture after sonication, add Triton X-100 to a final concentration of 1% and mix gently for 30 min at 4 °C with the mixer to increase the solubility of the protein.

The PierceTM Coomassie (Bradford) Protein Assay Kit (Thermo Scientific) was performed to determine the concentration of the protein. SDS-PAGE gel was used to determine the identity of the proteins by their expected molar mass and also to check for contaminates (if yes, there will be other bands shown together with the protein band). Furthermore, GST protein was run with the SDS-PAGE gel as a control. The gel was stained with Coomassie Blue and is presented in Figure 6-figure supplement 1 in (Zhen et al., 2016): Live-cell single-molecule tracking reveals co-recognition of H3K27me3 and DNA targets polycomb Cbx7-PRC1 to chromatin.

Notes

To monitor the OD, measure the absorbance at 600 nm. Estimate the amount of time by assuming the population of E. coli doubles every 20 min.

The cell pellet from step C10 can be frozen at -80 °C for several months.

For sonication: cell disruption is evidenced by partial clearing of the suspension. Avoid over sonication since it will heat the solution, leading to protein aggregation and denaturation.

After sonication, 1% Triton X-100 was added, Triton X-100 can be replaced with other detergents such as NP-40 or Tween 20.

This work was supported, in whole or in part, by the National Cancer Institute of the National Institutes of Health under Award Number R03CA191443 (to XR). This work was also supported by grants from the CU-Denver Office Research Service (to XR) and the American Cancer Society Grant IRG 57-001-53 subaward (to XR).
This protocol was modified from (Harper and Speicher, 2011).

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